The Hubble constant is used in astrophysics to characterize the expansion of the universe. The constant specifically relates the distance to a galaxy to that galaxyüs velocity of recession. Ironically, because gravitational forces depend on the distance between objects, the Hubble constant is actually a function of the expansion of the universe (i.e., it changes as the universe expands).

In 1929, American astronomers Edwin Hubble and Milton Humason (1891-1972), expanding upon the work of Vesto M. Slipher (1875-1969), discovered that light from distant galaxies is systematically increased in wavelength. This increase in wavelength, called a red shift, is denoted (z). Hubble interpreted observed red shifts as a consequence of the Doppler effect, which implied that the galaxies in his sample were all receding from the Sun with a velocity (v) determined by the equation, v = cz, where (c) is the speed of light. Of profound...